1. Field of the Invention
The present invention relates to a forming apparatus for sheet blanks such as sheet metals or plastics. In particular, the present invention relates to a multi-purpose and multi-functional forming apparatus which can form a sheet blank into a product of various curved surface shapes without using a die.
2. Description of the Background Art
In general, a matched die forming method by placing a sheet blank between female and male dies and pressing it with a press has been popularly used as a sheet forming method. Recently, a rubber pad forming apparatus, a hydroforming or fluid forming apparatus and a blow forming apparatus have been developed and used. The rubber pad forming apparatus applies a pressure to a sheet blank on a non-matched single die by means of a rubber and the hydroforming apparatus applies a hydraulic pressure thereto (refer to Metals Handbook, 9th edition, Vol. 14, ASM, Metals Park, Ohio, pp.605.about.615) and the blow forming apparatus heats a special metal sheet on the die and then deforms it by means of an air pressure (refer to Metals Handbook, 9th edition, Vol. 14, ASM, Metals Park, Ohio, pp.852.about.860). In addition, a multi-point contact forming apparatus has been developed and used, and this apparatus is provided with a group of steel rods or a matrix-type array of punches as a substitute for a matched-die on the upper and the lower sides of a blank sheet, and controls the height of each element of the punches so as to form a desired curved surface of the sheet (refer to `A Dieless Forming Technique for sheet metal` by Li Mingzhe et al. presented at International Conference in Seoul, Korea on Jul. 8, 1999). An excessively-large curved surface structure, such as a hull of a ship, may be manually formed, instead of using a forming apparatus.
However, in the conventional methods, a matched die method has disadvantages in that the design and manufacture of dies, especially large-sized dies, require a long term and high cost. Moreover, the dies should be replaced in accordance with the shape of products, and one-part forming process often involves several sets of dies. Additionally, in the case a die is damaged or seriously abraded, further expenses are incurred in repairing or re-fabricating it. Furthermore, the material, thickness and size of the blank for the matched die method is restricted.
As compared with a matched die method, a rubber pad forming apparatus, a hydroforming apparatus and a blow forming apparatus have advantages owing to the non-matched single die in that it is not necessary to use a sheet of a fixed thickness, and that it is quite easy to fabricate and modify the die. However, high expenses are also incurred in fabricating, repairing and modifying the die. Furthermore, in the case of a blow forming apparatus, a special material and a heating operation are required.
To conclude, in the conventional process of matched die forming, blow forming, rubber pad forming and hydroforming, some kinds of dies such as a pair of dies, several sets of matched dies or single die are used and replaced along with every product, which incur high expenses for dies and products. It also takes a long term, in most cases a few months or more, to design and fabricate the dies, and installation of the dies requires additional time whenever products change. Furthermore, the material, thickness and size of the blank sheets are often limited especially for the matched die process.
On the other hand, manual working does not incur expenses for a die, but the working time is quite long, so the productivity is low, and additionally, precision is deteriorated.
A multi-point contact forming apparatus using an array of steel rods or punches can reduce the expenses and time for fabrication and installation of the die, as compared with methods using a die, and has a higher productivity than manual working. However, because the group of steel rods or punches are double configured at the upper and lower parts, the price of the equipment is very high, at least 40% higher than that of the present invention. In addition, it is difficult to precisely control a height of each element of the steel rods or punches on the both sides at the same time. Moreover, a great force is locally applied to the ends of the steel rods or punches which ends are contacted to a point on both surfaces of the blank. Accordingly, especially in the case of a thin sheet, a wrinkle or a caved defect may be easily created at the contacting points of the sheet. In addition, only simple two- or three- dimensional curved surfaces can be formed, and deep drawing is almost impossible.
FIG. 1 is an illustration of force distribution on the punches during the conventional dieless forming process. As shown in FIG. 1 illustrating a distribution of force during the conventional dieless forming, some idle punches 2a which do not contribute to the forming occur, resulting in a low efficiency of the press and a low accuracy in forming. The compressive force is also highly concentrated on the contacting points of only non-idle punches 2b on the both sides of the sheet, making defects of dimple and wrinkle on the sheet. Thus, a pair of additional elastic spring pads are sometimes required at both sides between the punches and the blank sheet to prevent the defects, although, owing to their own bending limits, the pads restrict the limit of the curvature of the sheet formed. Moreover, as the contacting points of the opposite upper and lower punches are differently dislocated from the central point of the punches on the curved plane as shown in FIG. 1, precise adjustment of each element movement is so difficult that non-uniform deformation and wrinkle often occur in the area between the contacting points. Consequently, application area of the conventional dieless forming is so limited that sheet forming requiring large deformation such as deep drawing is nearly impossible, and only simple curved surfaces can be made.